Up until now we always redrew the whole windows, which worked
fine - except for some transparency issues - but left performance
on the table.
So now we use instancing to render only the actually damaged regions
inside the window, which also fixes our few rendering issues.
We also use a shader now for clearing the screen to use instancing as
well and lower the amount of draw calls at the begin of a frame for
clearing parts of it.
And while we are add it we remove the last rendering artifacts
by optimizing the damage vector by deduplicating and merging rectangles.
Victor Brekenfeld